1. Field of the Invention
The invention relates to apparatus and methods for treating spinal disorders. More particularly, the invention relates to spinal fixation systems.
2. Summary of the Related Art
The human spine is a system of articulated vertebral segments with tissues including vertebrae, intervertebral discs, facet joints, ligaments, and muscles. The human spine generally includes 24 vertebrae and the sacrum. These 24 vertebrae are designated from the head to the pelvis (cervical, thoracic, lumbar, and sacral). There are 7 cervical vertebrae, 12 thoracic vertebrae, 5 lumbar vertebrae (although this number may vary from 4 to 6 lumbar vertebrae in some humans), and 4 sacral vertebrae. The spine generally includes 25 articulations; each bone articulates with the one above and below. The superior C1 vertebra articulates with the skull and the inferior L5 vertebra articulates with the sacrum. With two exceptions, articulations between the vertebrae are through intervertebral discs and bilateral facet joints. The exceptions are the occipital-C1 and C1-C2 articulations. In addition to the vertebral discs and facet joints, other structural elements of the vertebrae include ligaments which connect and allow constrained mobility of the vertebrae, and musculature attachments through tendons to fixation points on the vertebrae to allow motion and maintain stability. The spine functions mechanically to protect the neurological elements of the spinal cord, to bear load and maintain posture, and to allow motion of the trunk and neck. Failure of any structural components of the spine as a result of disease or injury may lead to loss of the mechanical integrity of the spine, which may lead to neurological injury, mechanical instability, and deformity.
Loss of mechanical integrity of the spine may result from congenital or developmental abnormality, metabolic disorder, tumor, infection, trauma, arthritis, and degenerative disc diseaseor injury to any of the functional units of the spine, including vertebrae, intervertebral discs, facet joints, ligaments, and muscles. Ultimately, this can lead to pain, loss of function, and/or neurological impairment.
One of the main treatment modalities for loss of mechanical integrity of the spine has been the use of spinal fixation systems. These systems function to restore the mechanical integrity of the spine, by improving spine stability and correcting deformity.
A typical spinal fixation system includes 2 primary components: bone anchors and structural members. Bone anchors allow mechanical connection to vertebrae and may include, but are not limited to, such fixation means as screws, hooks, wires, and clips. Structural members allow interconnection between the bone anchors and they include, but are not limited to, such objects as rods or plates. The strategies for correcting mechanical instability and spinal deformity are varied, but typically allow for multiple points of fixation to the spine above and below the unstable segments or areas of deformity. Structural members are attached to these multiple points of spine fixation to the spine, providing mechanical stability and/or correction of deformity by supporting load and transmitting corrective forces and moments.
There are a variety of spinal fixation systems; for example, U.S. Pat. No. 5,176,680 discloses a device for fixing a spinal rod to vertebral screws, in which a spinal rod is passed through a split ring which is positioned between the prongs of a vertebral screw having a forked head. This assembly is locked into place by a locking screw threaded between the prongs of the forked head and onto the split ring. Similarly, U.S. Pat. No. 5,545,166 discloses a spinal fixation system that includes a plurality of anchor screws, clamp assemblies, pivot blocks, clamp blocks and rods that are implanted along a patient's spine to fix two or more adjacent vertebrae relative to each other. The mechanical functional requirements are varied over the spine. This results in variable loading and anatomic variation within the spine. This variation in load and anatomy has resulted in a large variety of sizes and types of anchor fixations to the vertebrae, as well as variation in size and strength of the structural members. The great variability in size and load carrying capacity of various portions of the spine has resulted in spinal fixation systems specifically adapted to the major areas of spine, such as cervical, thoracic, and lumbar vertebrae in both children and adults. Due to the high variability in anatomy and effect of disease, current systems include a large number of attachment members with customized rod and/or plate connectors, each of which is a unitary structure adapted to a particular area and application within the spine. Attachment and structural members adapted to one area of the spine are not readily adaptable for use in other areas of the spine. As a consequence, a very large inventory of different spine systems with specialized attachment members and connectors are needed for universal spine instrumentation. Therefore, there is a need for a new spinal fixation system that overcomes the lack of adaptability and universality of current spine systems comprised of functionally unique components requiring large inventories of various components that are often rarely used. Ideally, a spinal instrumentation system that allows universal fixation of all regions of the spine in either children or adults for a myriad of pathologic conditions and loading requirements with a minimum number of unique structural elements would reduce inventories, production, and shipping expenses.